This invention relates to an apparatus for and a method of operating a pair of ultrasonic transducers to produce a standing wave by the interaction of their outputs.
The use of the acoustic forces of ultrasonic standing waves in a fluid medium has been explored in a number of ways for controlling the movement of material suspended in the medium. To produce the standing wave, reliance has generally been placed on having the ultrasonic output of a single source directed through the medium in a chamber or other enclosed space and reflected back on itself from an end wall of the chamber, thereby producing an essentially stationary pressure pattern in a region of the chamber.
It would be desirable for many applications to give the standing wave a progressive movement, in addition or alternatively to any movement of the fluid medium. In GB No. 2 098 498A, for example, there are two opposed transducers driven by respective signal generators and a phase control of unspecified form is employed to control the phase angle between the signals from the signal generators to establish a movement of the standing wave pattern between the transducers for displacing particles transversely of a fluid flow.
In principle, the propagation of signals of different frequencies from coaxially opposed sources produces movement of the standing wave pattern along the axis of propagation at a rate dependent upon the difference between the two frequencies. In practice, to produce an appropriate rate of movement of the pattern for the control of suspended material will require a very small frequency difference because it is desirable to employ relatively high ultrasonic frequencies to control the movement of small suspended particles. This problem is intensified if it is required to produce relative movement between the different particle types having different acoustic properties, when the choice of frequency difference may be critical. To obtain a slow rate of progression of the standing wave pattern at a high ultrasonic frequency means that the change of phase per cycle is very small indeed and the problem then arises that the opposed sources can influence each other and it may be difficult or impossible to establish such very small phase changes if the two transducers tend to lock together.
It is an object of the present invention to provide an improved apparatus for and method of controlling a pair of ultrasonic transducers to produce a progressive standing wave through the interaction of their outputs.